In the field of a magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the floppy disc has been demanded.
In the field of a magnetic tape, with the prevalence of an office computer such as a minicomputer, a personal computer and a work station, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such the use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing ability (e.g., information throughput).
Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO.sub.2, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder, which are coated on a nonmagnetic support, have been conventionally widely used in magnetic recording media. Ferromagnetic metal fine powders and hexagonal ferrite fine powders among these have been known to have excellent high density recording properties.
In the case of a disc, as high capacity discs using ferromagnetic metal fine powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, neither of these are satisfactory with respect to capacities and properties. As is the circumstance, various attempts have been made to improve high density recording characteristics. For example, high capacity and high density recording of from 100 M to 200 M, e.g., LS-120 and ZIP, has been realized, and now even higher density recording of areal recording density of 0.15 G bit/inch.sup.2 or more has begun to be demanded. In addition, due to the requirement for shortening access time, the rotation frequency of discs has a tendency to increase.
In such a high density, high rotational and high transfer magnetic recording medium, higher running property and higher durability than those of conventional media are required for maintaining stable recording/reproduction.
Prior art techniques proposed for mainly improving recording density and running durability in coating type magnetic recording media are described below.
For example, JP-A-6-52541 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a magnetic tape having an average protrusion height of abrasives on the magnetic layer surface of 15 nm or less, and electromagnetic characteristics thereof are compatible with durability by improving head abrasion and head contamination.
U.S. Pat. No. 5,512,350 discloses a magnetic disc in which the magnetic layer has Ra of 15 nm or less, protrusions having a height of 30 nm or more are distributed from 125,000 to 250,000 per mm.sup.2 of the magnetic layer, and the amount of a lubricant in the magnetic layer is prescribed.
JP-A-6-309650 discloses a magnetic recording medium, in particular a magnetic disc, in which the magnetic layer contains from 8 to 30 parts by weight of a lubricant based on 100 parts by weight of the magnetic powder, and the number of protrusions having the height higher than the face which is 20 nm lower than the highest protrusion is specified to be from 400 to 2,500 per mm.sup.2, that is, the amount of the lubricant in the magnetic layer and the existing density of protrusions having specific height on the magnetic layer surface are specified to ensure the running stability.
Magnetic heads making electromagnetic induction as a principle of operation (an induction type magnetic head) have been conventionally used and prevailed. However, there are the limits to their abilities for further higher density recording/reproduction use. That is, it is necessary to increase the number of turns of a reproduction head coil to obtain higher reproduction output, but when the turn number of a reproduction head coil is increased, the inductance increases and the resistance at a high frequency region increases, as a result, reproduction output decreases.
A magnetic head making MR (magneto resistance) as a principle of operation has been proposed in recent years and is coming to be used in a hard disc, etc., and there is proposed in European Patent 729,135 the application thereof to a magnetic tape. An MR head can provide reproduction output of several times as large as that by an induction type magnetic head, and as no induction coil is used, equipment noise such as impedance noise is widely reduced, therefore, it has become possible to obtain a large S/N ratio by reducing the noise of a magnetic recording medium. That is, by reducing the noise of a magnetic recording medium naturally lurked in equipment noise, satisfactory recording/reproduction becomes feasible and high density recording characteristics can be strikingly improved.
However, there is a problem with an MR head such that noise is generated by the influence of a small amount of heat (thermal noise), in particular, if an MR head is touched to the protrusions on the surface of a magnetic layer, noise is suddenly and continuously increased, and in the case of digital recording, the problem is sometimes worsened in such a degree as error correction is impossible.
This problem of thermal noise is serious in a magnetic recording medium for use in a system of reproducing recorded signals of 0.15 G bit/inch.sup.2 or more of areal recording density, and it has been expected to be solved from the magnetic recording medium side.